1. Field of the Invention
This invention relates to an optical scanner and also to an image forming apparatus. More particularly, the present invention relates to an optical scanner adapted to deflect the light beam emitted from a light source and cause it to scan the surface of an object of scanning by way of an imaging optical system. Such an optical scanner can suitably be used for a laser beam printer (LBP) or a digital copying machine designed to form an image of an object by way of an electrophotographic process.
2. Related Background Art
Optical scanners to be used for image-forming apparatuses including laser beam printers and digital copying machines are adapted to cyclically deflect a light beam emitted from a light source and optically modulated in accordance with an image signal by means of an optical deflector such as a rotary polygon mirror, converge the light beam to a spot of light on the surface of a photosensitive recording medium (photosensitive drum) by means of an imaging optical system having a so-called fxcex8 feature and cause the light beam to scan the surface in order to record image information.
FIG. 1 of the accompanying drawings is a schematic perspective view of such a known typical optical scanner.
Referring to FIG. 1, the divergent light beam emitted from a light source 91 is substantially collimated by a collimator lens 92 and then its width is limited by an aperture 93. After passing through the aperture 93, the light beam is made to enter a cylindrical lens 94 having a predetermined refractive power only in the sub-scanning direction. The substantially collimated light beam entering the cylindrical lens 94 leaves the latter, keeping the substantially collimated condition in the main-scanning section (i.e. a plane intersecting it along the main-scanning direction). However, it is converged in the sub-scanning section (i.e. a plane intersecting it along the sub-scanning direction) and then focused on a deflecting surface (reflecting surface) 95a of an optical deflector 95, which is a rotary polygon mirror, to produce a substantially linear image. Then, the light beam deflected/reflected by the deflecting surface 95a of the optical deflector 95 is focused on the surface of a photosensitive drum 98 to be scanned by way of an imaging optical system (fxcex8 lens) 96 having an fxcex8 feature and a fold mirror 97 and made to optically scan the surface of the photosensitive drum 98 in the direction of arrow B (main-scanning direction) as the optical deflector 95 is rotated in the sense of arrow A in order to record image information thereon.
FIG. 2 is a schematic cross sectional view of a principal portion of the known optical scanner of FIG. 1 taken along the sub-scanning direction. In FIG. 2, the components same as those of FIG. 1 are denoted respectively by the same reference symbols and would not be described any further.
Referring to FIG. 2, the fold mirror 97 is typically secured to a holder member fitted to the optical cabinet or the main frame of the optical scanner. Such a holder member is reinforced to improve its rigidity and make it free from the vibrations produced by the motor of the optical deflector and that of the paper feeding system in the main frame. The fold mirror itself may be reinforced by means of a plate bonded thereto in order to improve its rigidity and shift its resonance frequency. However, it is highly difficult to completely eliminate the oscillation of the fold mirror by reinforcing the holder member and improving its rigidity.
FIG. 2 illustrates the movement of the scanning spot in the sub-scanning direction when the fold mirror 97 is oscillated in the direction of arrow C (direction of primary oscillation). When the incident light beam is deflected (reflected) by 90 degrees in the sub-scanning section, the scanning spot is moved on the surface being scanned (the surface of the image carrier which may be a photosensitive drum) 98 by an amount equal to the amplitude of the oscillation of the fold mirror 97. Then, as a result, the oscillation is visualized as an irregular pitch of scanning lines in the sub-scanning direction on the obtained image to remarkably degrade the image quality.
Therefore, it is the object of the present invention to provide an optical scanner and an image forming apparatus that can correct any displacement of the scanning spot in the sub-scanning direction due to the oscillation of the fold mirror and produce a high quality image that is free from an irregular pitch of scanning lines without resorting to costly anti-vibration means for the main frame and/or the optical cabinet of the optical scanner.
According to the invention, the above object is achieved by providing an optical scanner comprising:
a light source;
a deflection element for deflecting the light beam emitted from the light source;
a scanning optical system including at least first and second optical elements and adapted to focus the light beam deflected by the deflection element on the surface to be scanned; and
a reflector mirror arranged on the optical path between the first optical element and the second optical element to reflect the light beam coming from the first optical element and lead it to the second optical element.